The present invention generally relates to image transfer technology and, more particularly, to an apparatus and method for reducing contamination of image transfer surfaces of image transfer devices during the printing process, and an image transfer device having the apparatus.
As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in a liquid electrophotographic process, including laser printers, copiers, facsimiles, and the like.
In a liquid electrophotographic (LEP) printer, the surface of a photoconducting material (i.e., a photoreceptor) is charged to a substantially uniform potential so as to sensitize the surface. An electrostatic latent image is created on the surface of the photoconducting material by selectively exposing areas of the photoconductor surface to a light image of the original document being reproduced. A difference in electrostatic charge density is created between the areas on the photoconductor surface exposed and unexposed to light. In LEP, the photoconductor surface is initially charged to approximately ±1000 Volts, with the exposed photoconductor surface discharged to approximately ±50 Volts.
The electrostatic latent image on the photoconductor surface is developed into a visible image using developer liquid, which is a mixture of solid electrostatic toners or pigments dispersed in a carrier liquid serving as a solvent (referred to herein as “imaging oil”). The carrier liquid is usually insulative. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and toner. The photoconductor surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For LEP printers, the preferred embodiment is that the photoconductor surface and toner have the same polarity.
A sheet of paper or other medium is passed close to the photoconductor surface, which may be in the form of a rotating drum or a continuous belt, transferring the toner from the photoconductor surface onto the paper in the pattern of the image developed on the photoconductor surface. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some printer embodiments, the toner may first be transferred from the photoconductor surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper.
Charging of the photoconductor surface may be accomplished by an ionization device. Several types of ionization devices are known, such as a corotron (a corona wire having a DC voltage and an electrostatic shield), a dicorotron (a glass covered corona wire with AC voltage, and electrostatic shield with DC voltage, and an insulating housing), a scorotron (a corotron with an added biased conducting grid), a discorotron (a dicorotron with an added biased conducting strip), a pin scorotron (a corona pin array housing a high voltage and a biased conducting grid), or a charge roller. Each of these ionization devices generate ozone (O3), and nitric oxides (NOx), which if present in sufficient quantities, must be vented and filtered from the image transfer device.
An active flow of air through the image transfer device may be provided to ventilate and filter ozone and/or nitric oxides from the image transfer device. Although an active airflow through the image transfer device is sometimes required or desired for ventilation, airflow through or past the photoconductor surface is problematic in long term use of the photoconductor surface. In particular, active airflow is problematic because the airflow evaporates the submicron oil layer on the photoconductor surface and entrains oil vapors present above the oil layer, thereby effectively thinning the oil layer. The remaining oil layer includes residual materials such as charge directors and other dissolved ink components that have high molecular weight and do not easily evaporate. The thinned oil layer provides reduced buffering of the molecules of residual material against ion bombardment, UV exposure and ozone penetration. Therefore, the residual materials in the oil are more likely to react and polymerize on the photoconductor surface. Additionally, the dissolved residual material in the thinned oil layer is much closer to or beyond its solubility limit. This increases the chance for dissolved residual materials to drop out of solution and polymerize on the photoconductor surface.
The contaminating film of polymerized material on the photoconductor surface eliminates the ability to either form latent images of small dots on the photoconductor surface, or transfer small dots from the photoconductor surface to paper. As contamination of the photoconductor increases over time, the print quality of subsequently printed images is reduced, and the useful life of the photoconductor surface is shortened. The contamination problem is often referred to as old photoconductor syndrome (OPS).
Representations of prior art embodiments of charging apparatuses using ionization-type charging devices and having ventilation systems are schematically illustrated in FIGS. 2A–2B. In the charging apparatus 30 of FIG. 2A, an active ventilating airflow in the direction of arrows 71 is established by a suitable vacuum system 72. Fresh air is drawn into the chamber 96 containing the charging device (i.e., corona wire 90 and grid 92) from outside the charging apparatus housing 80, and passes through a small gap 73 (created by positioning pins 86) between the housing 80 and the photoconductor surface 22, and then through conductive grid 92. The ozone generated near the corona wire 90 is drawn through an opening 74 at the end of chamber 96 opposite photoconductor surface 22, and then to a filter system 75. Due to the airflow between the housing 80 and the photoconductor surface 22, the submicron oil layer on the photoconductor surface 22 evaporates such that the oil layer is thinned, and some oil vapor becomes entrained in the airflow.
Problems caused by the illustrated airflow include contamination of the charging device (both corona wire 90 and grid 92), and contamination of the photoconductor surface 22. The charging device and interior housing walls become contaminated as the oil vapor entrained in the airflow reacts with the ozone, energetic ions and UV light to polymerize, and then coats the corona wire 90, conductive grid 92 and housing walls with sticky material. The efficiency of the coated corona wire 90 is immediately reduced. Further, the contamination forces frequent cleaning and/or replacement of the corona wire 90, conductive grid 92 and housing. The photoconductor surface 22 becomes contaminated as the residual material in the thinned oil layer reacts with the ozone, energetic ions and UV light to polymerize on the photoconductor surface 22, or drops out of solution and polymerizes on the photoconductor surface 22, as described above.
In the charging apparatus of FIG. 2B, an active ventilating airflow in the direction of arrows 76 is established by a suitable vacuum system 72. Fresh air is drawn into the chamber 96 containing the charging device (i.e., corona wire 90 and conductive grid 92) from a plenum 77 at the end of chamber 96 opposite photoconductor surface 22. The airflow moves through opening 74, past corona wire 90 and toward photoconductor surface 22. After the flow of air moves through the conductive grid 92 and small gap 73, the air is drawn out at one or more outlets 78 adjacent the photoconductor surface 22, and then to filter system 75. The ozone generated near the corona wire 90 is thereby forcibly moved through the conductive grid 92 and against the photoconductor surface 22.
As the airflow passes through the small gap 73 between the housing 80 and the photoconductor surface 22, the submicron oil layer on the photoconductor surface 22 evaporates such that the oil layer is thinned, and some oil vapor becomes entrained in the airflow. The photoconductor surface 22 becomes contaminated as the residual material in the thinned oil layer reacts with the ozone, energetic ions and UV light to polymerize on the photoconductor surface 22, or drops out of solution and polymerizes on the photoconductor surface 22, as described above. The rate of residual material polymerization on the photoconductor surface 22 is further increased as ozone is actively pulled toward the photoconductor surface 22 by the airflow path, thereby increasing the chemical exposure of the oil layer on the photoconductor surface 22.
During the process of charging the photoconductor surface, it is desirable that the photoconductor surface is free of residual materials from previous printing cycles, such as toner, charge directors and other dissolved materials in the imaging oil. However, effectively cleaning the photoconductor surface of all residual materials is very difficult, and some amount of residual material inevitably remains on the photoconductor surface. Therefore, there is a need for an apparatus or method to lessen or eliminate polymerization of the residual materials and the resulting filming of the photoconductor surface.